1. Field of the Invention
The invention relates to improvements in devices known as half-turn taps and in particular to the ceramic discs used to manufacture such devices.
2. Description of the Prior Art
Taps are devices designed to regulate the flow of a liquid, usually water. Their main functions are to completely shut off the flow of liquid and to progressively establish the flow of the liquid, that is to say to regulate the flow from a minimum, shut-off position to a maximum "wide open" position.
Numerous devices have been developed with this aim. They can be classified into two main families, "mobile valve" taps and "ceramic disc" taps.
In "mobile valve" taps sealing is achieved by applying a rubber washer to a rigid fixed seat whose shape it matches. In this case the flow rate is regulated by progressively moving the washer away from the seat by means of a spindle so as to increase the fluid flow cross-section.
There are many disadvantages related to the design of this type of tap. For a given position of the operating spindle the flow rate varies with temperature; insufficient sensibility of the flow rate adjustment; risk of defective sealing due to premature aging of the rubber washer; and imprecision of the closed position.
"Ceramic disc" tape overcome some of the aforementioned disadvantages of "mobile valve" taps and are generally of a design which uses two discs made from alumina or other ceramic materials.
Each disc has one or more openings and an extremely flat surface (maximum shape defect is two interference fringes--helium emission ray, i.e. 0.589 um) that is highly polished (roughness coefficient Ra 0.25 um or less).
Depending on the type of "ceramic disc" tap, the openings may be similar in both discs or different. Each occupies a disc sector.
One disc is fixed, the other rotates. They are superposed with their flat surfaces is contact.
The flatness of these ceramic discs and their smoothness are sufficient for the contact between their surfaces to provide the necessary sealing.
One disc, usually the fixed disc, is connected at its unpolished rear surface to the end of the liquid feed line through the intermediary of a seal which provides the static sealing of the tap.
The other, mobile disc is rotated by a spindle coupled to a handle.
Angular displacement of the mobile disc relative to the fixed disc progressively superposes the openings to create one or more passages whose cross-section regulates the flow of liquid from a null value up to the maximum flow rate.
Numerous patents describe such taps, for example: EP-0.055.181, EP-0.296.945 and DE-2.805.038.
In most taps described or available on the market each disc has one or more openings, the geometry of which is defined by one or two straight line segments parallel to diameters of the disc and one or two circular arcs concentric with the periphery of the disc.
Depending on the type of tap, the fixed disc and the mobile disc may be identical or different with regard to the number and shape of the openings.
Discs with two openings generally have identical openings in the fixed disc and the mobile disc, quarter-circle shape and diametrally opposite each other.
The maximum opening of a tap fitted with these discs is obtained by 90.degree. rotation of the mobile disc relative to the fixed disc. For this reason this type of tap is called a "quarter-turn" tap.
Other discs with a single opening have different openings on each of the two (fixed and mobile) discs. Usually (see, for example, the aforementioned document EP-0.296.945) the fixed disc has a comma-shape opening on an angle more than 180.degree. but less than 270.degree. and the mobile disc has an opening subtending an angle just less than 90.degree.. The maximum opening of the tap equipped with these discs is obtained by around 270.degree. rotation of the mobile disc relative to the fixed disc. For this reason this type of tap is called a "three-quarter-turn" tap.
These three types of "ceramic disc" tap have advantages over their valve-type counterparts. Nevertheless, they have certain disadvantages such as non-linearity of the flow rate for the "quarter-turn" and "half-turn" types; imprecise adjustment on opening and closing the flow of liquid for the "quarter-turn" and "half-turn" taps; high noise associated with some operating positions of the "quarter-turn" and "half-turn" taps; insufficient flow rate in the case of the "three-quarter-turn" tap; a general lack of balance in the "half-turn" and "three-quarter-turn" taps due to the asymmetry of the flow of the liquid relative to the axis of the discs; serious water hammer effects for the "quarter-turn" and "half-turn" taps; and the fully open position is difficult to achieve in a single movement with the "three-quarter-turn" tap.
The invention is directed to alleviating the aforementioned disadvantages by improving linearity of the flow of the liquid on opening and closing as compared with "half-turn" and "quarter-turn" taps; reducing the lack of balance in "half-turn" and "three-quarter-turn" taps; and increasing flow rate in "three-quarter-turn" taps.